Microbial fuel cells (MFCs) convert chemical energy into electrical energy by the action of microbes on substrates such as glucose. Metabolism of glucose in living organisms can be divided into two separate chemical half-reactions. The first half-reaction is the oxidation of glucose to produce carbon dioxide, protons, and electrons:C6H12O6+6H2O→6CO2+24H++24e−and the second half-reaction is the reduction of oxygen to form water:6O2+24H++24e−→12H2O.
When the oxidation half-reaction and reduction half-reaction are combined, the overall reaction for metabolism of glucose is:C6H12O6+6O2→6CO2+6H2O.
The energy released in this reaction is used in living organisms to generate ATP, which serves as an energy carrier in cells to drive biochemical reactions. The energy is harnessed by coupling the generation of protons and electrons to enzymes that generate ATP.
If these electrons and protons are diverted from ATP generation in the cell, they can be used to power a fuel cell. FIG. 9 shows an example of a microbial fuel cell. The electrons generated by the microbes in the “microbe compartment” (the anode compartment, containing the anode and analyte) are collected by the anode and pass through an electrical connection to the cathode. The protons generated pass through a cation-permeable membrane (e.g., a Nafion membrane as in FIG. 9). Typically, oxygen is used as the final electron acceptor; as oxygen is reduced at the cathode, it combines with the protons passing from the analyte into the catholyte to form H2O. As the electrons pass through the electrical connection between the anode and cathode, they can do work, and thus the microbial fuel cell (MFC) generates useful electricity from the oxidation of glucose.
Microbial fuel cells can, in theory, use any resource for fuel that can be digested and metabolized by microbes. Accordingly MFCs have enormous potential as alternative energy sources, such as in the use of biomass to generate electricity. The performance of MFCs will require improvements, however, before practical use can be made of MFCs.
Extracting the protons and electrons from the microbes is often done by means of a mediator, such as thionine, methylene blue, or methyl viologen. The mediator diffuses into the microbial cell, is reduced by the electrons generated during oxidation of glucose, diffuses out of the microbial cell, and is oxidized at the anode. The mediator can continue to act as a shuttle for electrons. However, the kinetics of diffusion can limit the kinetics of the reaction, and the redox potential of the mediator can constrain the voltage generated.
Mediator-less microbial fuel cells have been developed, where microbes can transfer electrons directly to the anode without the need of a mediator molecule. However, microorganisms capable of transferring electrons directly to an electrode are relatively uncommon.
The instant invention describes microbial fuel cells with enhanced performance characteristics, as well as compounds that are useful for enhancing MFC performance, and methods for enhancing MFC performance. The compounds and methods disclose allow microbes to transfer electrons to electrodes in a microbial fuel cell without the need for a mediator to diffuse into and out of the microbes, by inserting compounds of appropriate structure into the microbial membrane and facilitating transfer of electrons across the cell membrane.
In a more general sense, the invention describes methods for enhancing charge transfer (such as electron transfer) from a microorganism, in order to increase the rate of transmembrane charge transfer from the microorganism, and/or in order to increase the electromotive force (EMF) of transmembrane charge transfer from the microorganism